It is known in the art to store large amounts of data in automated data storage libraries. Such libraries typically use data storage media that include magnetic disks, magnetic tapes, optical disks, and the like. As those skilled in the art will appreciate, the data retrieved from such storage media and storage libraries corresponds exactly to the data originally written to the storage media. The time to search the data written to a plurality of data storage media disposed in a storage library scales with the size and complexity of the library. As a general matter, the plurality of storage media must be searched serially.
In holographic information storage, an entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material. This is done by intersecting two coherent laser beams within the storage material. The first, called the data beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce—for example, a simple collimated beam with a planar wavefront.
The resulting optical interference pattern, of the two coherent laser beams, causes chemical and/or physical changes in the photosensitive medium: a replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium. When the stored interference grating is illuminated with one of the two waves that was used during recording, some of this incident light is diffracted by the stored grating in such a fashion that the other wave is reconstructed. Illuminating the stored grating with a data beam reconstructs the reference beam, and vice versa.
A rather unique feature of holographic data storage is associative retrieval, wherein imprinting a partial or search data pattern on the data beam and illuminating the stored holograms reconstructs all of the reference beams that were used to store data. The intensity that is diffracted by each of the stored interference gratings into the corresponding reconstructed reference beam is proportional to the similarity between the search pattern and the content of that particular data page. By determining, for example, which reference beam has the highest intensity and then reading the corresponding data page with this reference beam, the closest match to the search pattern can be found without initially knowing its address.
Unlike searching data written to non-holographic data storage media, the time required to search the data encoded in a volume holographic data storage medium does not scale with database size and complexity. Rather, searching an encoded holographic data storage medium is a substantially instantaneous process.
Applicants' method utilizes the desirable aspects of both holographic and non-holographic data storage media to facilitate searching very large write-once read many times, i.e. “WORM”, databases, such as for example and without limitation tax records, DNA sequences, and the like. Applicants' method writes data to a volume holographic data storage media and also to one or more non-holographic data storage media. Applicants' method then associates, for each of a plurality of search queries, a range of data storage locations in the non-holographic data storage media with a reconstructed reference beam, and forms a database association for each of the search queries, an observed search reference beam, and a range of storage addresses on the non-holographic data storage media.
When searching for data, Applicants' method first searches the holographic data storage medium to generate a search reference beam, and then determines a range of storage addresses to search in the non-holographic data storage media using Applicants' database.